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Bridges/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby Tim is building a bridge with toothpicks and gumdrops. MOBY: Beep. Moby jumps out from under the table and smashes the bridge with both arms. TIM: Why? Moby shrugs. MOBY: Beep. Tim reads from a typed letter. TIM: Dear Tim and Moby, why are there so many different kinds of bridges? From, Simon. Bridges are cool. They give us ways to get from one place to another. Images show different bridges as Tim describes them. TIM: They cross water, canyons and even link up buildings. They range from simple footbridges over ]o huge steel structures that support the weight of thousands of cars passing over them every day. Images show examples of the three types of bridges: beam, arched, and suspension. TIM: There are tons of different bridge designs but they're pretty much all just variations or combinations of three basic types: beam, arched, and suspension. Beam bridges are the simplest and weakest of the three designs. Arched bridges are stronger, and suspension bridges are the strongest of the three. The stronger the bridge, the more distance it can cover in a single span. An image shows the span of the suspension bridge. MOBY: Beep. Animations identify the spans of each of the three types of bridges. TIM: A span is the length of a single bridge segment between two vertical supports called piers. Images illustrate the three factors that determine the bridge’s design. TIM: Which design is used depends on factors like cost, how long the bridge needs to be, and how much weight it needs to bear. It's not always just a matter of choosing the strongest design. An animation shows traffic traveling across a long beam bridge. TIM: The longest bridge in the world is a thirty-eight-kilometer-long beam structure near New Orleans. Making an arched or suspension bridge that long would have been incredibly expensive, and totally unnecessary. MOBY: Beep. TIM: Well, all bridges have to deal with the forces of gravity, tension, and compression. Moby picks up a bowling ball from the table and drops it on the floor. An arrow pointing down illustrates gravity. TIM: Gravity is the force that pulls everything on the earth's surface towards its center. Moby squeezes in the ends of a loaf of bread. Two arrows pointing toward each other illustrate compression. TIM: Compression is a force that squeezes matter together. Material in compression gets shorter. Moby stretches out a rubber band. Two arrows pointing away from each other illustrate tension. TIM: And tension is just the opposite: it's a force that pulls matter apart. Moby's rubber band breaks and hits Tim in the face. TIM: Material under tension gets longer. When gravity pulls down on a bridge, it puts both tension and compression on the span. An animation indicates three forces. An arrow shows gravity pulling down on the center of the span, arrows pointing outward show the force of tension on the span, and arrows pointing inward show compression. TIM: The top of the span shortens under compression. If that force gets to be too great, the bridge will buckle. An animation shows the bridge buckling and breaking. TIM: And the bottom of it lengthens under tension. The bridge will snap if that force gets to be too much. An animation shows the bridge snapping and breaking. MOBY: Beep. TIM: Well, bridges work against these forces in one or two ways. Some bridges displace, or move, force from weaker areas to stronger areas. A simple beam bridge displaces force from the beam to its piers. An animation shows an arrow pointing to the weak area in the center of the bridge’s beam and two other arrows pointing to the piers which support the bridge. TIM: Arch bridges dissipate force, spreading it out over a wide area, so that no one spot on the bridge has to bear a whole lot of force. They also displace force to special piers called abutments. The shape of the arch does both of these things naturally. An animation shows how an arch bridge dissipates, or spreads out force. Arrows move from the center of the arch out to the sides and down the abutments. MOBY: Beep. Moby daydreams that he is a giant in a city, surrounded by skyscrapers. He sees an arched bridge and presses his finger down. The force makes the bridge begin to buckle. More arches appear and strengthen and straighten the span. Arrows show the direction and placement of the dissipated forces down the arches. TIM: Multiple arches spread the force out even more, letting you span greater distances. Trusses, linked triangular supports, are often added to beam bridges to dissipate force. An animation shows the trusses. Moby presses his finger on the center of the bridge. Arrows show tension and displacement forces moving downward and outward along the trusses. TIM: It's kind of weird to think that the downward force of weight can be spread out to a truss above the bridge, but that's just what happens. An image shows a truss below a bridge. TIM: You sometimes see trusses below the roadway, too. An animation shows vehicles traveling over a suspension bridge. TIM: In a suspension bridge, the deck hangs from a series of cables, which are firmly anchored to the ground on either side of the span. Moby pushes down on the bridge. An animation shows how forces are distributed in a suspension bridge. TIM: The force of tension is transferred into these cables, while compression is transferred to a set of towers that are set firmly into the earth. Moby plucks the cables which sound like a harp. TIM: Moby? Moby? Are you okay? Moby is still daydreaming. MOBY: Beep. In his dream, Moby makes threatening sounds and topples some buildings. Laser beams shoot from his eyes and set buildings on fire. There’s a big explosion and then the screen turns red. TIM: Moby? Category:BrainPOP Transcripts Category:BrainPOP Engineering & Technology Transcripts